This invention relates to a device for straight traveling stabilization and change of attitude on predetermined paths for vehicle axles, particularly for industrial vehicles.
The known solutions to the problem of steering in the field of industrial vehicles, assuming the existence of a front axle with three sectors or the like, with steerable wheels controlled by a steering wheel, substantially comprise a steering system with driven axles or a self-steering system with self-steering axles.
The steering system with controlled axles finds application only in the case of vehicles with two or more steerable front axles and a central axle controlled by steering the first axle; it cannot normally be used for axles which are considerably spaced from the first one, for obvious technical reasons.
The solution of the correct steering on a determined path, with several controlled axles, is always particularly onerous, above all in its maintenance, and has various drawbacks so that it is resorted to only in particular cases and for special vehicles. Except for a few rare exceptions, it is not used in large production.
The main drawbacks of this system are the high production cost which justifies its adoption only in particular cases, the difficult and costly maintenance, the low mechanical efficiency of the transmissions, the difficult adjustment and permanent keeping of the angle values, the hazardous reliability because the breaking or malfunction of the elements of one axle will detrimentally affect the whole steering and consequently the running of the vehicle, and the fact that in case of breaking of the servomechanisms the excessive resistance is prejudicial to the manual action of the driver.
The self-steering system with self-steering axles finds today application in the construction of trailers and semi-trailers, in converting vehicles from two axles to three axles and in vehicles with three or more axles for high speed duty. In vehicles with two driving axles, steering problems are sacrificed for adhesion with the result of heavy drag on the rear axles and movement on only partially determined paths.
As the self-steering axle works on the principle of the action of external friction forces on an axle composed of three sections and said forces are transmitted to pivoting wheels connected by a tie rod, the operation necessarily involves an initial dragging step and a final dragging step the duration of which depends on the inertia of the system.
Further, in case of differences in the transverse plane of the external forces acting on the single wheels (unevenness of roadbed between one wheel and the other, holes, differences in the road level, heavy transverse gradients, etc.), the intensity of dragging increases in the initial and final steps and does not cease completely even in the central steering step.
In any case in self-steering axles there are steering angles which depend on external factors such as the coefficient of friction, distribution of load, swinging of suspensions and transverse gradients.
The self-steering system with self-steering axles further requires the insertion of transversely acting shock absorbers whose instability due to effects of wear or external forces makes the steering path indeterminate.
A further important drawback of this system is the necessity of having a locking device for going in reverse because the wheels of the self-steering axle tend to go into the opposite direction to that of the steerable wheels. Thus, for going in reverse two operations are required: one in forward movement for bringing the wheels parallel to the longitudinal axis of the vehicle and one on the locking device.
It is thus evident from the foregoing explanations that the problem of steering of multi-axle vehicles has not yet found a perfect solution and remains in the field of approximations and the defects which do not affect directly the safety of driving are prejudicial to road holding and wear the tires due to dragging which has not been completely eliminated.
It is an object of the present invention to provide a servo steering device which, when applied on vehicle axles, permits the conventional concept of the steering axle to be reversed by designing said axle initially as a fixed axle with the possibility of controlled steering instead of "always steering". In this manner obviously advantages would be obtained over the known steering systems as the steering axle would be freed from the action of the external forces which are unknown and cannot be exactly predetermined, and would instead be subject to internal forces determined in advance.
More particularly, it is an object of the present invention to provide a servo steering device which would permit the steering axle to:
be independent of the coefficient of friction;
provide always determined paths and eliminate dragging;
in case of breakdown, not influence the main steering system and in this case behave as a fixed axle;
secure alignment;
define the path according to the angles required by a correct turn.